Multi-zone furnaces are widely used in the metals industry, (e.g. copper and steel industries) to meet process heating (or re-heating) requirements. For example, a multiple zone reheat furnace is used in the copper and steel industries to reheat copper and steel billets and ingots for further processing. Typically, a continuous reheat furnace with two or more zones is employed in the steel industry to ensure not only the even distribution of heat, but also the precise control of temperature. These furnaces consume an enormous amount of energy, ranging from 20 to 400 million BTU per hour. Emissions in the form of NOx associated with such enormous use of energy in these furnaces has a profound impact both on the environment and the cost competitiveness of the user. Because of the serious impact on the environment, these industries have been faced with a challenge, namely, reducing the overall emission of NOx to the environment while controlling the operating cost at the same time, to remain competitive in a Global market.
Numerous approaches have been tried in the metals and other industries such as the glass and power generation industries to control and/or reduce the generation and/or emission of NOx. These approaches can be classified into three separate and broad categories: pretreatment, combustion modification, and post-treatment, as described by C. E. Baukal author of Pollutant Emissions, Chapter 3, Oxygen-Enhanced Combustion, CRC Press, 1998. Numerous pretreatment approaches such as switching fuel, changing oxidizer, using fuel additives, treating fuel, etc. have been tried to control and/or reduce the generation and/or emission of NOx with some or limited success. Likewise, numerous post-treatment approaches (generally aimed at removing NOx present in the combustion flue gas with scrubbing or chemical treatment) have been developed and commercially used these days by the power generation and chemical process industries. For example, NOx reduction approaches based on injecting ammonia or urea such as selective non-catalytic reduction (SNCR) and selective catalytic reduction (SCR) have been successfully developed and employed by the power generation industry to treat flue gases from very large furnaces or power plants. These post treatment approaches are not only very capital intensive, but they also significantly increase operating costs. They are, therefore, not suitable or not economical to treat flue gases from small to medium size furnaces. Consequently, they are not widely employed for controlling NOx emission from multi-zone reheat furnaces employed by the copper and steel industries. Finally, numerous combustion modification approaches have been developed to implement changes that interrupt the process of NOx formation. Some of these approaches include flue gas recirculation, staged or pulsed combustion of fuel, and advanced mixing. Among the above three broad categories, the combustion modification approaches appear to be most promising and economically attractive to control or reduce NOx emission from small to medium size furnaces.
A number of combustion modification approaches known in the literature can be used to reduce NOx emission from reheat furnaces. For example, a gas reburn approach described in U.S. Pat. No. 5,756,059 can be applied to control NOx emission from a continuous reheat furnace, though it has not yet been used to control NOx from reheat furnaces. According to this patent, NOx emission from a multi-zone continuous steel reheat furnace can be controlled by removing a portion of fuel from one or more combustion zones and injecting it downstream of the combustion zones using an array of specially designed nozzles. The NOx formed in the combustion zones is reacted with the fuel injected downstream of the combustion zones, thereby reducing the overall level of NOx in the flue gas. However, this process results in increasing the level of carbon monoxide in the flue gas. Additional air or oxidant is, therefore, injected using specially designed nozzles to react with carbon monoxide before sending the flue gas out of the furnace to a heat recuperator. Although the reburn approach is technically sound in order to control NOx emission from continuous reheat furnaces, it is very difficult to implement in a retrofit furnace environment. It requires drilling multiple holes in existing reheat furnaces to inject fuel and oxidant, a furnace modification that is neither desirable nor acceptable to operators of reheat furnaces.
Another combustion modification approach that can be used to reduce NOx emission from continuous re-heating furnaces has been described in U.S. Pat. No. 5,203,859. Although the patent describes a process for reducing NOx emission from a glass melter, it can be adapted to a continuous reheat furnace. According to this patent, the stoichiometry of oxygen-enriched combustion in the primary combustion zone (or re-heating zone) is modified to make it fuel rich, thereby reducing the generation of NOx in the primary combustion zone. An additional oxygen-enriched air stream is injected through an array of nozzles downstream of the primary combustion zone (or re-heating zone) to combust the remaining fuel and CO produced in the primary combustion zone. Once again, the process disclosed in this patent is technically sound for control of NOx emission from continuous reheat furnaces. However, it is very difficult to implement this technique in a commercial metal treating furnace. Again the furnace operator must drill multiple holes in the existing reheat furnaces to inject air or oxygen-enriched air. Such furnace modifications are neither desirable nor acceptable to operators of reheat furnaces.
Other combustion modification approaches that can be used to reduce NOx emission from continuous re-heating furnaces are described in U.S. Pat. Nos. 5,569,312, 5,573,568, 5,849,059, and 5,851,256. Although these patents describe a process for reducing NOx in a regenerative glass melter, it can be adapted to a continuous reheat furnace. According to these patents, the treatment to reduce NOx is carried out completely outside the furnace. Specifically, fuel is injected at the entrance of the regenerator located outside the furnace to react with NOx formed in the furnace. The un-reacted fuel and carbon monoxide thus produced is reacted with air that is injected further downstream in the regenerator. The processes disclosed in these patents, therefore, require drilling arrays of holes in the regenerator to inject fuel and air, a modification of the regenerator that is neither desirable nor acceptable to operators of reheat furnaces.
One can incorporate various combustion modification approaches and come up with new burners (low NOx burners) that will by itself produce low NOx. In order to use these low NOx burners, one needs to remove all existing burners and replace them with new low NOx burners. Since reheat furnaces typically use more than 20 burners, replacing old burners with new low NOx burners requires substantial capital investment which is neither desirable nor acceptable to operators of reheat furnaces.
Based on the above discussion, it is clear that a number of technologies are currently available to control NOx emission from a multi-zone furnaces provided one is willing to intrusively modify such furnaces (or drilling holes in furnaces) for fuel and/or air injection, or invest significant amounts of money to replace old burners with new low NOx burners. Therefore, there is a need to develop a method for control of NOx emission from multi-zone furnaces without replacing burners or intrusively modifying such furnaces for additional fuel and/or oxidant injection and that is simple to implement.